Rechargeable alkaline batteries typically require separators to function primarily as dielectric as well as electrolyte reservoirs. In addition to being inert to 31% potassium hydroxide (KOH) used as the electrolyte, separator materials should possess durable wettability to withstand the rigors during the discharging and recharging of the battery. Typical separator constructions employ nonwoven structures comprising either nylon or polyolefin fibers.
Representative examples of such constructions are illustrated in the following patents.
U.S. Pat. No. 5,389,471 to Kung discloses a separator for an alkaline battery comprised of a porous sheet of a microporous film, fabric or synthetic paper which sheet is saturated with a resin containing one or more carboxyl groups neutralized with a base so as to form a salt. The resin includes a high molecular weight acrylic acid having one or more carboxyl groups. A particularly preferred resin disclosed in the examples is Carbopol (a high molecular weight acrylic acid homopolymer).
U.S. Pat. No. 5,439,734 to Everhart discloses a nonwoven fabric formed from polyolefin blended with at least one di-fatty acid ester hydrophilic additive. The additives in Everhart include a dioleate ester of polyethylene oxide, ethoxylated ester of caster oil, a blend of glycerol mono-oleate ester and ethoxylated nonylphenol and Maypeg—400 ml monolaurate.
Palmer U.S. Pat. No. 3,847,676 teaches a battery separator made of a non woven mat of fibers of polymeric resin, i.e., C2–C8 polyolefin thermoplastic such as polyethylene, polypropylene and polystyrene containing a first wetting agent dispersed therein (relatively water insoluble), the fibers having a coating on their outer surfaces of a second wetting agent. The internally dispersed wetting agents are surfactants and preferably C8 to C18 phenol surfactants having 1–15 moles of ethylene oxide. The second wetting agent which is coated on the exterior of the fibers is relatively water soluble and relatively oil insoluble and is preferably an anionic and/or nonionic surfactant.
Palmer U.S. Pat. No. 3,870,567 deals with battery separators formed from nonwoven thermoplastic fiber mats, the fibers containing an internal wetting agent that will bloom over a period of time at ambient temperatures of the battery. Nonylphenol ethylene oxide is an example of a suitable wetting agent.
Palmer U.S. Pat. No. 3,918,995 also involves a battery separator produced from a nonwoven mat of plastic fibers having an internal surfactant and a second surfactant coated on the exterior of the fibers. This patent is a division of Palmer U.S. Pat. No. 3,847,676, the claims in the '995 patent being drawn to a battery as contrasted with a separator.
Palmer U.S. Pat. No. 3,933,525 discloses nonwoven battery separators comprised of polyolefin fibers having internal wetting agents, preferably comprised of two surfactants. The preferred surfactants are C8 and C18 phenol surfactants having 1–15 moles of ethylene dioxide.
Broadhead U.S. Pat. No. 3,928,067 discloses polypropylene separators for use in lithium non aqueous secondary batteries which include as wetting agents for the polypropylene separators polyalkylene glycol esters, tetraalkylammonium halides and certain lithium salts and preferably combinations of certain polyalkylene glycol ethers and tetralkyl-ammonium halides.
Bunton U.S. Pat. No. 3,947,537 discloses battery separators made from nonwoven mats of thermoplastic fibers which have been wetted with a surfactant water mixture of an anionic surfactant such as an aliphatic sulfate or a non-ionic surfactant such as a polyethylene oxy compound.
EP 0 450 449 B1 is directed to separator materials for storage batteries comprising a fabric sheet made of sulfonated conjugate fibers comprising at least first and second components thermally bonded together, the first component being a surface layer, the second component occupying a core portion, the first component being an ethylene copolymer containing at least a unit having the formula —CH2—C(SO3H) (COOH)— and comprising an ethylene carbonic acid monomer containing acrylic and/or nucleic acid and possibly also an acrylic acid ester, the second component comprising a non-sulfonated polyolefin.
EP 0 591 616 B1 is directed to a hydrophilized separator material of a nonwoven composed of a mixture of polyamide and/or polyolefin fibers of different softening ranges, characterized in that the separator material is moistened with deionized water before its use.
EP 0 680 107 B1 is concerned with a nickel-hydrogen secondary battery including a separator formed of a sheet material containing polyolefin based synthetic resin fibers having a first surface which is hydrophilic and a second surface having a hydrophilic portion and a hydrophobic portion. The polyolefin fibers are core-sheath type composite fibers comprising a core of polyolefin and a sheath of another polyolefin.
EP 0 710 994 A2 relates to a battery separator comprising (1) a nonwoven web of fibers having a mean diameter of about 15 μm or less and (2) a graft polymerized monomer on the surface of the nonwoven web which renders the nonwoven web wettable.
EP 0 756 340 A1 is directed to a battery separator comprising a nonwoven web of first and second fibers, the first fibers corresponding to a first and second polyolefin, the second fibers comprising a third polyolefin, treated so that the separator preferably formed of two such nonwoven webs, is spontaneously wettable by an electrolyte.
EP 0 795 916 A1 relates to a wet-laid nonwoven fabric formed from three dimensional entanglement of thermoplastic staple fibers with hot melt fibers suitable for use as a battery separator. The resultant nonwoven fabric can be subjected to a hydrophilic treatment with a “generally used surfactant, a sulphonation treatment, a fluorination treatment, a plasma treatment or a corona discharge treatment.
EP 0 834 938 A2 discloses an alkaline battery separator formed by heat fusion and hydroentangling 1) polyolefin dividable composite fibers 2) high strength composite fibers (polypropylene) and 3) polyolefin heat sensitive adhesive fibers, all as more specifically defined. This application also teaches treatment of the resultant fabric for imparting a hydrophilic property by employing a sulphonating treatment, a treatment with fluorine gas, a graft polymerization treatment with vinyl monomers, a treatment with a surface active agent, a treatment used to adhere hydrophilic resins, a discharging treatment, or the like. As surface active agents there are disclosed anionic surface active agents (alkali metal salt of a higher fatty acid, alkyl sulfonate, or a salt of sulfosuccinate).
WO98/31060 discloses a battery separator useful in batteries of the recombinant or sealed type made from extremely fine meltblown fibers self-bonded in a cohesive, uncompressive mass. This fiber mat is made wettable by battery acid by addition of a surface active agent to the polymer prior to extrusion or by covalently bonding hydrophilic groups to the surface of the fibers after formation. Suitable additives are polytetrahydrafuran, mono & diglycerides from fatty acids & dimethylsilicone oxyalkylene copolymer.
WO99/00447 discloses a product and process for making wettable fibers prepared from an olefin polymer, polyester or polyamide including a wetting agent consisting essentially of a monoglyceride or a combination of a monoglyceride and a mixed glyceride with the monoglyceride amounting to at least 85% by weight in the case of the combination.
The monoglyceride corresponds to the formula

wherein —OR1, OR2, and —OR3 are hydroxyl or a fatty acid ester group, but only one of them is a fatty acid ester group (C12-22) The mixed glyceride (di- or tri-) corresponds to the formula

wherein —OR4, OR5, and —OR6 are hydroxyl or a fatty acid ester group (C12-22). The combination of this di- or tri- glyceride with the monoglyceride constitutes the wetting agent in accordance with one embodiment.
As is shown in the prior art both nylon and polyolefins have inherent property limitations, which lead to shorter battery life. While nylon is susceptible to alkaline degradation, polyolefins, though being chemically inert, are hydrophobic in nature.
It is known in the art to convert polypropylene fiber, which is hydrophobic, into a hydrophilic fiber by chemically modifying its surface. However, topical chemical applications are not entirely satisfactory as they are not durable, and other types of surface modifications may need extra processing steps and tend to be expensive. In addition, some of these modifications age with time, especially in the presence of 31% potassium hydroxide (KOH) electrolyte solution used in alkaline batteries. The few processes known to render the polyolefins wettable are environmentally unfriendly, very slow processes and are not durable enough.
An alternative and improvement over chemical modification is to directly melt blend a hydrophilic additive into the polypropylene or thermoplastic polymer rendering the fibers themselves hydrophilic. The invention solves the forgoing problems and provides such a product by incorporating one or more hydrophilic melt additives into polyolefin resin (polypropylene (PP) or polyethylene (PE) or bicomponent) fibers to produce nonwoven constructions for use as battery separators. The hydrophilic melt additives are incorporated into PP polymer fiber which is then converted into nonwoven separator materials by wet laid and carding/thermal bonding processes. Alternatively, the PP polymer and additives may be converted directly from the polymer into nonwoven form by spunbonding or meltblowing, or a combination of the two.
The preferred melt additives are an admixture of hydroxy phenols and polyethylene glycols. The hydroxy phenol is characterized in that it contains the functional group HOC6H4—. According to the preferred embodiments of the invention, the nonwoven battery separator is fabricated employing wet laid and carded thermal bonding processes. An advantage of the invention is obtained by use of combinations of hydrophobic and hydrophilic fibers in the battery separator fabric, i.e., all fibers in the separator need not be permanently wettable. In the preferred embodiment, the separator includes bicomponent fibers in which the melt additive is incorporated into the sheath constituent(s) of the fiber. Use of bicomponent fibers, as well as combinations of hydrophobic and hydrophilic fibers, reduces costs and permits optimization of the separator for diverse applications.
A broad aspect of the invention is to provide a nonwoven web that is durable and has the wettability and strength for use in rechargeable alkaline batteries by directly incorporating melt additives into the polymeric component during melt processing to form a wettable fiber matrix. This fiber matrix can be meltblown, spunbonded or made into staple fibers to form a 100% wettable web. Alternatively the wettable fiber matrix can be mixed with binder fibers that are wettable or non-wettable or mixtures of both which are then made into a nonwoven web.
An object of the invention is to provide a nonwoven web with increased wettability and strength for use as battery separator material.
Another broad object of the invention is to provide a nonwoven that is durable and wettable in harsh environments.
A further object of the invention is to provide a nonwoven web that has both hydrophilic and hydrophobic regions.
Another further object of the invention is to provide a method for producing products that can be designed to have varied wettablility and strength properties depending on the desired end use applications.
A specific object of the invention is to provide a lower cost battery separator material including bicomponent fibers, wherein melt additives are incorporated in the fiber sheath of the bicomponent fiber and not the core.
Another specific object of the invention is to provide an economical battery separator material made of both wettable and non-wettable polymeric fibers.
A more specific object of the invention is to provide a nonwoven web that can be used for other applications such as diapers and feminine care products, and medical applications which would require durable wettability.
Another object of the invention is to provide a nonwoven web that can be used in clothing applications, wherein products produced remain durable and hydrophilic after multiple machine washings.
Another object of the invention is to provide a nonwoven that can be used in filtration applications, wherein durable and wettable properties are required.